The southern Appalachian hinterland is composed of a
number of terranes with distinctly different geologic histories.
Some terranes, such as the central and northern Virginia Blue
Ridge, are clearly of North American or Laurentian affinity
(Rankin and others, 1989; Horton and others, 1989). Others,
such as the Carolina slate belt, are demonstrably exotic with
respect to Laurentia (Secor and others, 1983; Hibbard and
others, 2002). Still others, such as the Goochland and
Chopawamsic terranes, are, because of a lack of definitive
evidence, of uncertain affinity and considered to be “suspect”
with respect to Laurentia. While it is generally understood
that these exotic and suspect terranes were accreted to the
Laurentian margin during Paleozoic orogenesis, major unresolved
questions remain, including (1) the origin and affinity
of such terranes, (2) the timing of their accretion, and (3) the
kinematics of deformation along terrane boundaries.

In central Virginia, at the northern end of the southern
Appalachians, exposed terranes include, from west to east,
the Blue Ridge, the Western Piedmont, the Chopawamsic, the
Goochland, and the Southeastern Piedmont (fig. 1). They are
separated from each other by major fault systems along which
multiple generations of motion are recognized. The present
state of these boundaries holds clues to the past relations of
the adjacent terranes. Understanding the affinity of these terranes
and the kinematics and timing of their juxtaposition is a
prerequisite for accurately describing the tectonic history of
the Virginia Piedmont.

The Goochland and Chopawamsic terranes have markedly
different geologic histories (fig. 2). The Goochland terrane,
in the eastern Piedmont, has been described as a Middle to
Late Proterozoic basement massif with relict granulite-facies
mineral assemblages (Glover and others, 1982; Farrar, 1984;
Farrar and Owens, 2001). The Chopawamsic terrane, in the
central Piedmont, is an Ordovician volcanic-plutonic arc
complex (Pavlides, 1981; Coler and others, 2000). The
boundary between the Goochland and Chopawamsic terranes
coincides with a pronounced northeast-trending aeromagnetic
and aeroradiometric feature known as the Spotsylvania lineament.
Neuschel (1970) first recognized this geophysical
boundary before mapping, geochronology, and tectonic models
provided an adequate framework for understanding its significance.
Later workers described this discontinuity as a brittle
thrust fault (Pavlides and others, 1980a), a mylonite zone
(Farrar, 1984; Brown, 1986), and a “major suture” (Marr,
1991). Recent workers have named this zone the Spotsylvania
high-strain zone (Spears and Bailey, 2002; Bailey and others,
in press).

The purpose of this field trip is to focus on new research
within and along the boundary between the Goochland and
Chopawamsic terranes in central Virginia. We present new
evidence describing the kinematics of deformation in the
Spotsylvania high-strain zone with indications of large-scale
relative displacement between the terranes. Surprising new
geochronology in the Goochland terrane challenges longstanding
assumptions about its history. We also examine two
enigmatic fault slices and a suite of unusual mafic to ultramafic
igneous rocks and speculate as to their origin. This
work provides new insights into the spatial and temporal relation
of the terranes, with implications for the tectonic assembly
of the Piedmont.

Geology

Goochland Terrane

The Goochland terrane is composed of multiply
deformed and metamorphosed gneiss, amphibolite, granite,
and anorthosite (fig. 2). The oldest and structurally lowest
unit in the Goochland terrane is the State Farm Gneiss
(Brown, 1937), a coarse-grained granitic gneiss that crops out
in a series of domes (fig. 3) that are overlain by the Sabot
Amphibolite and the heterogeneous Maidens Gneiss (Poland,
1976). Both the Sabot and the Maidens are intruded by the
Montpelier Anorthosite (Aleinikoff and others, 1996). U-Pb
zircon analyses of the State Farm Gneiss and Montpelier Anorthosite yield Mesoproterozoic
ages of 1,050 to 1,020 Ma
(Aleinikoff and others, 1996; Owens and Tucker, 1999). A
suite of small A-type granitoid plutons with U-Pb zircon ages
of ~630 Ma intrudes the State Farm Gneiss (Owens and
Tucker, 2000). The Maidens, the most extensive map unit in
the Goochland terrane, is dominantly pelitic (biotite-garnet
and muscovite-sillimanite gneiss) with some granitic gneiss.

Rocks of the Goochland terrane experienced an early
granulite-facies metamorphic event that was overprinted by a
later amphibolite-facies event (Farrar, 1984; Farrar and
Owens, 2001). Farrar (1984) interprets the early granulite-facies
metamorphism as Mesoproterozoic and the amphibolite-facies event as Alleghanian (~300–250 Ma). The origin of
the Goochland terrane is unclear; the Mesoproterozoic rocks
and A-type Neoproterozoic granitoids are similar to
Laurentian basement in the Blue Ridge (Glover and others,
1978; Farrar, 1984; Glover and others, 1989; Aleinikoff and
others, 1996). New Nd-isotopic results reported by Owens
and Samson (2001, in press) for the State Farm Gneiss and
Montpelier Anorthosite show that these units are isotopically
similar to other blocks of Mesoproterozoic crust along the
eastern and southern margins of Laurentia (for example,
Adirondacks, Blue Ridge, Llano uplift); however, other workers
have suggested that the Goochland terrane may be of peri-Gondwanan affinity (Rankin and others, 1989; Hibbard and
Samson, 1995).

Previously, the entire Goochland terrane was considered to
be a coherent block of Mesoproterozoic crust. Mesoproterozoic
crystallization ages based on modern U-Pb zircon methods have
been confirmed for both the Montpelier Anorthosite (1,045±10
Ma; Aleinikoff and others, 1996) and the State Farm Gneiss
(~1,046–1,023 Ma; Owens and Tucker, 2003). In addition,
Horton and others (1995) reported a U-Pb zircon age of
1,035±5 Ma for a granitic gneiss within the Maidens Gneiss
near Amelia Courthouse, possibly indicating that the Maidens is
also Mesoproterozoic. However, new results based on electron microprobe dating
of monazite in more typical Maidens lithologies
(metapelites, and so forth) have thus far revealed no ages
older than about 420 Ma (R.J. Tracy, B.E. Owens, and C.R.
Shirvell, unpub. data). If these monazite ages reflect the timing
of granulite-facies metamorphism (a plausible interpretation),
the long-held assumption that the high-grade event was
Grenvillian is clearly incorrect (see Burton and Armstrong,
1997). Furthermore, new U-Pb zircon results for a probable
metaigneous variety of Maidens Gneiss (Stop 3) indicate a
Paleozoic age, suggesting that at least some portions of the
Maidens Gneiss are younger than Mesoproterozoic. An interesting
additional point in this regard is that Neoproterozoic granitoids
have thus far not been recognized within the Maidens
Gneiss; in other words, they appear to be restricted to the State
Farm Gneiss (Owens and Tucker, 2003). These points suggest
the possibility of a previously unrecognized unconformity or
structural discontinuity between at least the western part of the
Maidens Gneiss and the more easterly (State Farm, Sabot, and
Montpelier) portion of the Goochland terrane.

Chopawamsic Terrane

The Chopawamsic terrane is composed of metamorphosed
volcanic and sedimentary rocks with a suite of associated
granitoid plutons, all of Middle to Late Ordovician age
(fig. 2). The most widespread unit is the Chopawamsic
Formation, a suite of mafic and felsic metavolcanic rocks
dated at ~470 Ma (Horton and others, 1998; Coler and others,
2000). In central Virginia, the Chopawamsic is intruded by
the Columbia pluton (fig. 3), a granite to granodiorite body
that yielded a U-Pb SIMS (secondary ion mass spectrometry)
zircon age of 457±7 Ma (Wilson, 2001). In the western part
of the Chopawamsic terrane, both the Chopawamsic
Formation and the Columbia pluton are unconformably overlain
by the Arvonia Formation (figs. 2, 3), a metasedimentary
package that contains Late Ordovician fossils (Darton, 1892;
Watson and Powell, 1911; Tillman, 1970). The northeastern
part of the terrane contains a similar metasedimentary unit,
the Quantico Formation, which may be partly interlayered
with the Chopawamsic Formation. Late Ordovician fossils
also are present in the Quantico Formation (Pavlides and others,
1980b).

Rocks of the Chopawamsic terrane preserve evidence of
one regional metamorphic event. Metamorphic mineral
assemblages indicate that greenschist-facies conditions were
reached along the northwest side of the terrane; these grade
into amphibolite-facies assemblages in the southeast part of
the terrane. Metamorphic hornblende from the Chopawamsic
Formation dated by 40Ar/39Ar methods yielded ages of
318 to ~284 Ma (Burton and others, 2000). The Chopawamsic terrane
is interpreted to be an Ordovician volcanic arc complex
developed on continental crust outboard of Laurentia (Coler
and others, 2000) and later accreted during the Late
Ordovician Taconic orogeny (Glover and others, 1989).

Spotsylvania High-Strain Zone

The Spotsylvania high-strain zone (SHSZ) forms the
boundary between the early Paleozoic Chopawamsic terrane
and the Mesoproterozoic-Paleozoic(?) Goochland terrane in
the central Virginia Piedmont (fig. 3). This boundary was
originally recognized as a sharp geophysical (aeromagnetic
and aeroradiometric) lineament (Neuschel, 1970) and interpreted
as a brittle fault. In the Piedmont of southern Virginia,
the SHSZ appears to connect with the Hyco shear zone, a
component of the Alleghanian-age central Piedmont shear
zone, a major boundary traceable for over 500 km (kilometers;
300 mi (miles)) in the southern Appalachians (Hibbard
and others, 1998; Wortman and others, 1998). Hibbard and
others (1998) interpreted the Hyco zone in southern Virginia
to be a ductile thrust that emplaced the Carolina terrane over
the Chopawamsic terrane. Farrar (1984), Pratt and others
(1988), and Glover and others (1989) interpreted the
Spotsylvania zone as a significant thrust fault (not a suture)
along which granulite/amphibolite-facies rocks of the
Goochland belt were emplaced to the northwest in the late
Paleozoic. In north-central Virginia, Pavlides and others
(1980a) interpreted the Spotsylvania zone to be a 2- to 3-km
(1–2 mi)-wide zone of predominantly brittle en-echelon
faults. In central Virginia, Marr (1991) reported the presence
of a tectonic mélange zone within the SHSZ and suggested it
may represent a suture. Bourland (1976) and Spears and
Bailey (2002) recognized brittle fault rocks in the SHSZ and
interpreted these to have formed during Mesozoic reactivation
of the Paleozoic high-strain zone. The Spotsylvania zone is
located within the central Virginia seismic zone (Bollinger
and others, 1986; Çoruh and others, 1988) and as recently as
2003, small earthquakes have occurred at depth along this
structure.

We define the SHSZ as a ~15-km (~9-mi)-wide belt of
heterogeneously deformed mylonitic rocks that typically lacks
distinct boundaries (fig. 3). Its northwestern boundary is
defined by the geophysical lineament at the contact between
amphibole-rich gneisses of the Elk Hill Complex to the northwest
and mylonitic rocks derived from more granitic to pelitic
protoliths to the southeast. Gneissic rocks to the southeast of
the Spotsylvania lineament are strongly deformed well into
the Goochland terrane. Mylonitic biotite schist, granitic
mylonite, biotite-rich ultramylonite, amphibolite, and protomylonitic
pegmatite are the most common rock types in the
SHSZ. Foliation in the SHSZ strikes to the northeast and generally
dips moderately to gently to the southeast. At some
locations, where SHSZ is located in the hanging wall of
listric Mesozoic normal faults, dips of mylonitic foliations
flattened out due to horizontal axis rotations associated with
normal faulting. Lineations (both elongation and mineral)
plunge shallowly to the northeast and southwest in the plane
of the foliation. Asymmetric porphyroclast tails and boudins
from surfaces normal to foliation and parallel to lineation
consistently exhibit a strike-parallel dextral asymmetry across
the SHSZ. Pegmatite dikes are commonly folded and boudinaged
in a geometry consistent with bulk constrictional strain
(K>1) (Bailey and others, in press). Folded dikes are asymmetric;
the folds generally verge to the northwest. The geometry
of asymmetric structures, both parallel and normal to
the elongation lineation, is consistent with a modest triclinic
deformation symmetry (Bailey and others, in press).

Minimum sectional strains, estimated from boudinaged
and folded dikes on lineation-parallel surfaces, range from
8:1 to >20:1. Feldspar porphyroclasts, pegmatitic boudins,
and amphibolite boudins are superficially similar to clasts or
blocks in a mélange, but exhibit consistent dextral asymmetries
and at many locations occur as tabular bodies with a
pinch-and-swell character (Stops 3, 4, 8). Backward-rotated
porphyroclasts are common in SHSZ ultramylonites and vorticity
analysis yields Wn-values between 0.8 and 0.4, indicating
general shear deformation that significantly deviated from
simple shear (Bailey and others, in press).

Quartz grains in mylonitic rocks from the SHSZ are
completely recrystallized, straight extinction is common, and
strong crystallographic preferred orientations are well developed.
Feldspar porphyroclasts display core and mantle structures
and strong undulose extinction. In mylonites and ultramylonites,
myrmekite and flame perthite are localized along
high-strain grain boundaries. Synkinematic metamorphic minerals
include biotite, garnet, epidote, and staurolite.
Microstructures preserved in mylonitic rocks from the SHSZ
are consistent with deformation conditions in the upper greenschist
to lower amphibolite facies (450–500°C).

In order to better constrain the kinematics and tectonic
significance of the SHSZ, Bailey and others (in press) used
estimated values for vorticity and three-dimensional strain to
restore the Goochland terrane to its paleogeographic position
prior to dextral transpression. Deformation in the SHSZ produced
significant thinning (~40–70 percent) normal to the
zone and up to 500 percent stretching parallel to the zone
boundaries. With the Chopawamsic terrane fixed in position,
the Goochland terrane is retrodeformed to a predeformation
position 80 to 300 km (50–186 mi) northeast of its present
location. These displacement estimates are minimum values
because strains were calculated from boudinaged and folded
dikes that are, in themselves, minimum strain indicators.
Furthermore, the Brookneal/Shores high-strain zone and the
Mountain Run fault zone, more westerly structural discontinuities
in the Virginia Piedmont (fig. 1), also exhibit dextral
motion (Gates and others, 1986; Bobyarchick, 1999). Thus,
the Goochland terrane, relative to the more western elements
in the Virginia Piedmont, experienced significant southwestern
translation during the Alleghanian orogeny.

Fault Slices of Uncertain Affinity Associated with the Terrane Boundary

Two narrow belts of rocks originally described by Taber
(1913) are now recognized to be fault-bounded blocks of
unknown affinity (Spears and Bailey, 2002). Although well
documented by Taber (1913) and Brown (1937), the pegmatite
belt and Elk Hill Complex were excluded from map
compilations in the second half of the twentieth century (for
example, Virginia Division of Mineral Resources, 1993). We
find that both blocks exist as mappable fault-bounded units
spatially associated with the terrane boundary. No published
geochronology exists for any of the rocks in these two fault
blocks. Comparison of these rocks to the Goochland and
Chopawamsic terranes does not yield obvious correlations to
units in either of the adjacent terranes.

Pegmatite Belt

Taber (1913) used the term “pegmatite belt” to describe
an area underlain by pegmatite and granite in western
Goochland and northern Cumberland Counties. Some later
workers (Jonas, 1932; Brown, 1937) honored Taber’s nomenclature
and included a similar area of pegmatite on their geologic
maps. However, the 1963 geologic map of Virginia
(Virginia Division of Mineral Resources, 1963) depicts this
area as an extension of the Columbia granite. Farrar (1984)
recognized pegmatite in this area and interpreted it to be associated
with the intrusion of the Columbia pluton. However, on
the 1993 geologic map of Virginia (Virginia Division of
Mineral Resources, 1993), this area was mapped as biotite
gneiss with small intrusions of biotite granite, all within the
Chopawamsic terrane.

We find that these rocks are lithologically distinct and
separated by faults from the Chopawamsic Formation and the
Elk Hill Complex (fig. 3). The recently described Little Fork
Church fault, mappable by a lithologic discontinuity coincident
with both ductile and brittle fault rocks (Spears and
Bailey, 2002) forms the western boundary of the pegmatite
belt (fig. 3). The eastern boundary is defined by the Lakeside fault, which
separates the pegmatite belt from the Elk Hill
Complex (Stops 6, 10) (fig. 3). The belt can be separated into
three distinct lithologic packages from west (structurally lowest)
to east (structurally highest). The structurally lowest unit
(pgg) is composed of light-gray, fine-grained, weakly layered
micaceous granitic gneiss with abundant white to pink potassium
feldspar-quartz-muscovite pegmatite (Stop 8; fig. 4).
The pegmatite is concordant with the foliation in the surrounding
gneiss and is commonly deformed into lens-shaped
domains containing potassium feldspar porphyroclasts in a
fine-grained matrix of muscovite, quartz, and microcline.
Large feldspars are commonly kaolinized and display
throughgoing brittle fractures. The middle unit (pga, fig.
4) is
composed of weakly layered granitic gneiss similar to the
lower unit, with less pegmatite, and generally concordant
bodies of fine-grained amphibolite that are commonly
deformed into boudins (Stop 10C). The upper unit (pgl) is
strongly compositionally layered amphibolite, biotite gneiss,
and minor pegmatite (Stop 10B, fig. 4).

Elk Hill Complex

The Elk Hill Complex was named by Taber (1913) for
exposures in cuts along the railroad southeast of Elk Hill plantation
in western Goochland County. At its type locality, the
Elk Hill is dominated by strongly compositionally layered
hornblende gneiss with lesser amounts of biotite gneiss and
pegmatite. We find that, in addition to these lithologies, the Elk
Hill contains gneissic diorite, talc-chlorite soapstone, and, especially
south of the James River, distinctive phenocrystic felsic
rocks resembling pinkish, fine-grained granite in outcrop.

Brown (1937) recognized the Elk Hill Complex on his
geologic map of Goochland County, but the name was excluded
from the literature for the rest of the 20th century. A hornblende
gneiss unit was indicated in this area on the 1963 geologic
map of Virginia (Virginia Division of Mineral Resources,
1963); however, on the 1993 geologic map of Virginia
(Virginia Division of Mineral Resources, 1993), rocks in this
area were mapped as “biotite gneiss” contiguous with the
Central Virginia volcanic-plutonic belt, which at that time was
a synonym for the Chopawamsic terrane.

Our work demonstrates that the Elk Hill Complex is distinct
and separated by faults from both the Chopawamsic and
the Goochland terranes. The Lakeside fault, previously
mapped from the early Mesozoic Farmville basin northeastward
to a point just south of the James River (Virginia
Division of Mineral Resources, 1993), in fact extends farther
northeastward across the James River and across western
Goochland County at least as far north as I-64. This fault separates the Elk
Hill Complex from the pegmatite belt and the
Chopawamsic Formation throughout the area mapped. The
eastern boundary of the Elk Hill is marked by the strongly
mylonitic rocks of the Spotsylvania high-strain zone.

Although the Elk Hill Complex and the Chopawamsic
Formation are superficially similar in that they are both dominated
by mafic metavolcanic rocks, we note certain dissimilarities.
The Elk Hill includes, particularly south of the James
River, distinctive fine-grained felsic metavolcanic rocks interlayered
with amphibolite. The felsic rocks contain concentrically
zoned plagioclase phenocrysts, presumably of primary
volcanic origin; such phenocrysts are not observed in the
Chopawamsic Formation at this latitude. Geophysically, the
Chopawamsic Formation is characterized by a high-amplitude,
short-wavelength pattern on total intensity aeromagnetic
maps; the pattern over the Elk Hill Complex is lower amplitude
and longer wavelength. Furthermore, the Chopawamsic
Formation contains substantial deposits of precious metals
and massive sulfides. These well-known deposits were heavily
exploited beginning in the early 19th century, to the point
that a well-defined band of rocks now recognized as the
Chopawamsic terrane was known as the “gold-pyrite belt”
(Lonsdale, 1927; Spears and Upchurch, 1997). Despite
intense prospecting by gold seekers, adjacent parts of the
Piedmont remained largely unproductive. The Elk Hill (as
well as the pegmatite belt and the Goochland terrane) is
apparently barren of metallic mineralization, as demonstrated
by the total absence of known mines.

These dissimilarities raise suspicions that the Elk Hill
Complex and the Chopawamsic Formation, while both ostensibly
of volcanic origin, may be of different ages and affinities.
Additional work is needed to fully characterize the differences
between these two blocks, and to establish the possible
relation of the Elk Hill to other metavolcanic units in the
Piedmont.

Conclusions

The Goochland and Chopawamsic terranes have markedly
different histories that indicate that they developed independently
and were not juxtaposed until post-Late
Ordovician. Unpublished monazite ages in the Goochland terrane,
referred to above, raise the intriguing possibility that
they were separate until even later, post ~420 Ma (Silurian),
and that the granulite-facies metamorphic event may be middle
Paleozoic. While the basement rocks of the Goochland
terrane, including the State Farm Gneiss, superficially resemble
Laurentian rocks of the Virginia Blue Ridge, our work on
the kinematics of its western boundary indicates that it originated
far to the north. Quantitative understanding of the kinematics
does not resolve whether the Goochland is a native
Laurentian or an exotic terrane; however it does place meaningful
limits on its pre-Alleghanian position in the
Appalachian orogen. If the Goochland terrane is Laurentian,
it originated somewhere between the Pennsylvania reentrant
and the New York promontory, not outboard of the Virginia
Blue Ridge.

The Elk Hill Complex and the pegmatite belt form two
fault slices of uncertain affinity between the Goochland terrane
and the Chopawamsic terrane proper. In addition, we
recognize an unusual suite of mafic to ultramafic rocks associated
with faults along the terrane boundary. Further work is
needed to establish the significance of these units and their
relation to adjacent terranes. These previously unrecognized
crustal elements must be considered in future models for the
tectonic assembly of the southern Appalachian Piedmont.

Acknowledgments

This manuscript benefited greatly from a review by Bill
Burton. Amy Gilmer assisted with conversion and drafting of
figures. We thank the many landowners who provided access
to outcrops.

Bailey, C.M., Francis, B.E., and Fahrney, E.E., in press,
Strain and vorticity analysis of transpressional high-strain
zones from the Virginia Piedmont, USA: Geological
Society of London Special Paper.

Bourland, W.C., 1976, Tectonogenesis and metamorphism of
the Piedmont from Columbia to Westview, Virginia, along
the James River: Blacksburg, Virginia Polytechnic Institute
and State University, M.S. thesis, 105 p.